Piezoelectric materials were first discovered in the 1800s. Since then, manufacturing technology advances have led to the demand for clean-rooms and a complex process that makes films and blocks that are attached to electronics after machining. The costly process and the intrinsic brittleness of the material, has restricted the ability to maximize the material's potential.
The piezoelectric materials that are in everything from cell phones to musical greeting cards may be getting an upgrade thanks to work detailed in the journal Nature Materials released online Jan 21. Xiaoyu ‘Rayne' Zheng, an assistant professor of mechanical engineering in the College of Engineering at Virginia Tech, and a member of the Macromolecules Innovation Institute, and his team have devised techniques for 3D printing piezoelectric materials that can be custom designed to convert movement, impact, and stress from any directions into electrical energy.
"Piezoelectric materials convert strain and stress into electric charges," Zheng explained. The above image, shows a printed flexible sheet of piezoelectric smart material (Photo by H. Cui of the Zheng Lab).
Zheng's team developed a set of 3D printable topologies that let them manipulate and design arbitrary piezoelectric constants, which result in the material generating electric charge in response to incoming forces and vibrations from any direction. Unlike conventional piezoelectrics in which the inherent crystals prescribe the electric charge movements, the new method lets users dictate and program voltage responses to be magnified, reversed, or suppressed in any direction.
Flexible and customizable piezoelectric material developed at Virginia Tech
"We have developed a design method and printing platform to freely design the sensitivity and operational modes of piezoelectric materials," Zheng said. "By programming the 3D active topology, you can achieve pretty much any combination of piezoelectric coefficients within a material, and use them as transducers and sensors that are not only flexible and strong, but also respond to pressure, vibrations and impacts via electric signals that tell the location, magnitude and direction of the impacts within any location of these materials."
Specifically, they additively produced free-form, perovskite-based piezoelectric nanocomposites with complex three-dimensional architectures. Then, they were able to customize the material's resulting voltage response via selective suppression, reversal or enhancement with applied stress in any direction. Furthermore, they were able to achieve high specific piezoelectric constants and tailorable flexibility using just a fraction of the parent materials.
Piezoelectric Material Structure and Manufacturing
One factor in current piezoelectric fabrication is the natural crystal used. At the atomic level, the orientations of atoms are fixed. However, Zheng's team has created a substitute structure that mimics the crystal but allows altering and customization of the lattice orientation.
"We have synthesized a class of highly sensitive piezoelectric inks that can be sculpted into complex three-dimensional features with ultraviolet light. The inks contain highly concentrated piezoelectric nanocrystals bonded with UV-sensitive gels, which form a solution - a milky mixture like melted crystal - that we print with a high-resolution digital light 3D printer," Zheng said.
The team successfully demonstrated the 3D printed materials at a scale measuring fractions of the diameter of a human hair.
"We can tailor the architecture to make them more flexible and use them, for instance, as energy harvesting devices, wrapping them around any arbitrary curvature," Zheng said. "We can make them thick, and light, stiff or energy-absorbing."
The material boasts sensitivities 5-times as high as flexible piezoelectric polymers. The stiffness and shape of the material can be tuned and produced as a thin sheet that approximates a strip of gauze, or as a stiff block.
"We have a team making them into wearable devices, like rings, insoles, and fitting them into a boxing glove where we will be able to record impact forces and monitor the health of the user," said Zheng.
"The ability to achieve the desired mechanical, electrical and thermal properties will significantly reduce the time and effort needed to develop practical materials," said Shashank Priya, associate VP for research at Penn State and former professor of mechanical engineering at Virginia Tech.
Demonstrations of the Piezoelectric Material
So far, the team has printed and demonstrated smart materials wrapped around curved surfaces, as well as worn on hands and fingers to convert motion, and harvest the mechanical energy, but the potential applications go well beyond consumer electronics and wearables. Zheng sees a potential for the technology in robotics, tactile sensing, energy harvesting, and intelligent infrastructure. In such applications, a piezoelectric material could enable the sensing of impacts, vibrations, and motions, and it allows these types of movements to be monitored and located.
Assembled piezoelectric structural sensors
The team has printed a small smart bridge to demonstrate its ability to sense the locations and magnitudes of dropping impacts, while it is robust enough to absorb the impact energy.
The team also demonstrated their application in a smart transducer that converts underwater vibration to electric voltages.
"Traditionally, if you wanted to monitor the internal strength of a structure, you would need to have a lot of individual sensors placed all over the structure, each with a number of leads and connectors," said Huachen Cui, a doctoral student with Zheng and first author of the Nature Materials paper. "Here, the structure itself is the sensor - it can monitor itself."
